1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) baseband receiver capable of establishing synchronization with peripheral base stations in high precision at the time of a peripheral cell searching operation.
2. Description of the Related Art
In such an asynchronous DS-CDMA communication system, a plurality of base stations using a same frequency band are provided for cells adjacent to each other, respectively. When the base station communicates with a mobile station, the base station transmits a double-spread signal by use of a short code having a repetitive data symbol period and being common to the base stations, and a long code having a repetitive period longer than the repetitive data symbol period and being peculiar to the base station.
In this case, when the mobile station present in a cell moves to the adjacent cell, and when the mobile station carries out simultaneous communications with a plurality of base stations, it is necessary to conduct peripheral cell searching operation to establish synchronization with the long code peculiar to the base station disposed in the adjacent cell.
Conventionally, in the above peripheral cell searching operation, simply, a long code having the maximum correlation value of the detected long codes of peripheral base stations is detected and identified as the long code of the adjacent base station.
FIG. 1 shows the structure of a conventional CDMA baseband receiver. As shown in FIG. 1, the conventional CDMA baseband receiver is mainly composed of a short code generating unit 60, a correlating unit A 61, a maximum correlation peak phase detecting unit 62, a long code generating unit 63, a correlating unit B64, a threshold value evaluating unit 65, spreading code generating units A 66-1 to 66-m, known base station delay profile generating units 67-1 to 67-m, a fundamental path detecting unit 68, demodulation correlating units (1) 69-1 to (k) 69-k, spreading code generating units B 70-1 to 70-k, and a RAKE synthesizing unit 71.
Now, referring to FIG. 1, the operation of a peripheral cell searching operation by the conventional CDMA baseband receiver will be described.
A received signal is converted into a spread modulation signal as a baseband signal in a front stage circuit, which is not shown in the figure, and is inputted to a CDMA baseband receiver. The short code generating unit 60 generates a symbol code having a data symbol period as a short code, which is common to base stations. The correlating unit A 61 carries out correlation detection of the inputted spread modulation signal by use of only the short code generated by the short code generating unit 60 during N periods as a long code period. The maximum correlation peak phase detecting unit 62 detects Q correlation values from the maximum of correlation values outputted from the correlating unit A 61 to store the correlation values and the phases thereof. The phases of the Q correlation values from the maximum are used as long code phase candidates of peripheral base stations.
The long code generating unit 63 generates Q long codes corresponding to the Q long code phase candidates outputted from the maximum correlation peak phase detecting unit 62. The correlating unit B64 calculates correlation values between the spread modulation signal and spreading codes obtained by multiplying the short code outputted from the short code generating unit 60 by the Q long codes outputted from the long code generating unit 63 to obtain the correlation values of an A kind of long codes. The threshold value evaluating unit 65 determines a maximum one, which is equal to or greater than a predetermined threshold, of the correlation values outputted from the correlating unit B64 as the long code of an adjacent base station. Synchronization with the base station can be established by use of the determined long code.
Meanwhile, the spreading code generating units A 66-1 to 66-m multiply the long code, peculiar to the base station, which has been determined by the threshold value evaluating unit 65 by the short code to generate m kinds of spreading code. The known base station delay-profile generating units 67-1 to 67-m determines correlation values of n chip periods including the long code from the m kinds of spreading code generated by the spreading code generating units A 66-1 to 66-m and the spread modulation signal to generate and store a delay profile for m base stations from which signals are received.
The fundamental path detecting unit 68 detects the correlation peak phases of the P correlation values from the maximum delay profiles for the m base stations. The spreading code generating units B 70-1 to 70-k multiply the long code peculiar to the base station which has been determined by the threshold value evaluating unit 65 by the short code to generate k kinds of spreading code. The demodulation correlating units 69-1 to 69-k calculate and generate k correlation outputs between the P correlation peak phases from the fundamental path detecting unit 68 and the spreading modulation siganl, by use of the k kinds of spreading code outputted from the spreading code generating units B 70-1 to 70-k, respectively. The RAKE synthesizing unit 71 synthesizes the k correlation ouputs outputted from the demodulation correlating units 69-1 to 69-k to generate a demodulation signal of a digital signal. The demodulation signal is used to reproduce digital data bits in a rear stage circuit, which is not shown in the drawing.
In the peripheral cell searching operation carried out by the conventional CDMA baseband receiver, synchronization with the base station is established by use of a correlation peak phase obtained through correlation detection carried out using only the short code.
However, when the phases of the top Q correlation values are detected through the correlation detection carried out using only the short code common to the base stations, the correlation peak phase of a known base station is also detected, including a multi-path propagation component. In the peripheral cell searching operation of the conventional CDMA baseband receiver, it is impossible to remove the correlation peak phase of the known base station including the multi-path propagation component. Therefore, the identification of the long code is likely to be carried out by use of the long code phase of the known base station.
As mentioned above, when the long code phase of the known base station is used to carry out the long code identification, not only precision is deteriorated in establishment of synchronization with a peripheral cell, but also a searching time is likely to increase.
In addition, when the top Q correlation values are detected through the correlation detection carried out using only the short code, there is a possibility that the phases of all the detected correlation values are the correlation peak phases of known base stations. In this case, even if an appropriate peripheral base station is present, it is impossible to establish the synchronization with the base station.
In conjunction with the above description, a CDMA communication system is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 10-126378). In this reference, a base station (2) spreads and modulates a data signal using a long code and a short code. A transmitting section (13) transmits a spread modulation data signal in which the data signal is spread and modulated by only the short code for every predetermined position of the long code. A mobile station (1) receives the spread modulation data signal known base stations. In this case, even if an by a receiving section (4). A short code identifying section (7) of a control processing section (6) carries out an identifying process of the short code. A long code identifying section (8) carries out an identifying process of the long code based on the short code identifying process timing. Reception levels (correlation values) in a group for same timing of the long code are stored in a reception level table (10). A received spread modulation data signal is subjected to a despreading and demodulating operation using a long code with a maximum reception level.
Also, a DS-CDMA communication receiver is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 10-126380). In this reference, a correlation with a short code of a control channel is detected by a matched filter (22) in a searching operation. Also, a correlation peak position of the maximum power is detected as a long code timing. Next, correlating units (28-1 to 28-n) provided in parallel for RAKE synthesis determines a long code which is expected to the system, at the detected long code timing. After establishment of long code synchronization, a multi-path signal is received using the correlating units (28-1 to 28-n) to determine data through the RAKE synthesis. The identification of a long code for a peripheral cell as a candidate is carried out using the matched filter (22) in the peripheral cell searching operation. A signal from a communicating base station is received using the correlating units (28-1 to 28-n) and realizes hand-over at safety.
Also, a DS-CDMA communication receiver is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 10-200447). In this reference, a baseband reception signal is supplied to a matched filter (1) to calculate a correlation with a spreading code from a spreading code generating unit (2). A signal power calculating section (3) calculates power of correlation values outputted from the matched filter (1) to output a long code synchronization timing determining section (4), a threshold value calculating section (5), and a long code identifying section (6). A spreading code generating section (2) short code #0 common to control channels of the base stations in an initial cell searching operation. After a long code synchronization timing is determined, each piece of an N chip as a part of a spreading code sequence as a synthesis code of a long code #i peculiar to the base station and a short code #0 is outputted while being replaced.
Also, an initial synchronization capturing method is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 11-122141). In this reference, a transmitting station sets one or both of kind and phase of a long code peculiar to the transmitting station at an optional timing after a mask block in the mask block (a) and transmits the mask block. A receiving station demodulates the mask block to acquire one or both of kind and phase of the long code for a channel to be connected and the optional timing. Thus, the initial synchronization capturing method is attained.
Also, a spreading code synchronizing method is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 11-196460). In this reference, it is supposed that s(0) to s(3) are correlation detection output vectors obtained through correlation detection between a code obtained by multiplying a long code and a common short code and a reception signal for intervals (correlation detection interval 1) #0 to #3 of the reception signal. The phase is rotated in so based on data (+1 or −1 in this example) of a symbol. Therefore, when a vector summation of s( ) is calculated, the correlation is cancelled between the symbols so that high precision correlation detection cannot be carried out. Therefore, a correlation is calculated between the reception signal and the common short code at the reception timing of the mask symbol B, and a data modulation component and an amplitude and phase fluctuation due to fading are removed from s( ) using the correlation detection output (despread signal) vector p(B).